As well-known in the art, an ultra-high speed broadband communication system at a level of Gbps has been required nowadays to deal with an increased amount of data. For example, 60 GHz millimeter-wave band that meets such requirement has drawn a lot of interests. In order to apply such system to a mobile communication terminal, some features such as lower-power consumption, isolation between circuits, and a compact size should be satisfied.
The term of ‘isolation’ is the criteria showing how much two objects are isolated from each other, and is used as the parameter indicating the degree of inhibition of intrinsic signals of each port from going to the other ports because of the occurrence of interference in every port with respect to an I/O port to or from which the signals are inputted or outputted. For example, since a mixer having three ports such as RF, LO and IF is designed to achieve frequency conversion, it is essential to have port-to-port isolation.
In particular, a resistive frequency mixing apparatus used in a millimeter-wave band in a radio communication system has a structure that an input RF signal is converted into an IF signal depending on an input LO frequency by using FET (Field-Effect Transistor), LO filter, RF filter, or IF filter.
FIG. 1 shows one example of a conventional resistive frequency mixing apparatus, and FIG. 2 presents another example of a conventional resistive mixing apparatus proposed by Chalmers University of Technology in Sweden.
In the resistive frequency mixing apparatus illustrated in FIG. 1, an LO frequency is inputted to a gate of FET 12 via an LO matching circuit 10, an RF signal is applied to a drain of the FET 12 via a matching network including the matching circuit 10 and an RF filter 14, and an IF signal is outputted from a drain of the FET 12 via a matching network including the matching circuit 10 and an IF filter 16.
On the other hand, in the resistive frequency mixing apparatus illustrated in FIG. 2, branch line couplers (or baluns) 21 and 22 are provided at the input ports to receive RF and LO signals in order to improve isolation between RF and LO signals. By these branch line couplers 21 and 22, these input signals are divided into two paths. Therefore, LO frequencies are inputted to gates of two FETs 23 and 24, respectively, via LO filters (not shown). The RF signals separated through the branch line couplers 21 and 22 are connected to drains of the two FETs 23 and 24 via the RF filters 25 and 26, and IF signals are outputted from the drains of the two FETs 23 and 24 via IF filters 27 and 28.
However, since the resistive frequency mixing apparatus as shown in FIG. 1 has the RF input port and the IF output port connected to the drains of the FETs, it is difficult to secure the isolation between the RF and LO signals. Another resistive frequency mixing apparatus as shown in FIG. 2 requires a number of couplers (or baluns) and filters to secure the isolation, thus increasing the size of circuit and requiring relatively large LO power consumption to drive such circuit.